Development of imaging techniques of tissue characterization by means of time domain microwave computed tomography

1. 研究结论：探讨了微波成像在时域测量技术中的可行性。在本研究中使用时域测量技术来区分在两个天线之间的直线路径上传播的波分量和散射或反射波分量。这是一种类似于CP-MCT的技术，其中两个天线之间的直线路径是通过使用啁啾雷达的频域测量技术来区分的。研制了时域微波计算机断层扫描实验系统（TD-MCT），该系统由矢量网络分析仪和微波扫描仪组成，测量频率范围为0.8 GHz ~ 4.2 GHz的S21。两个天线之间的传输时间可以是时域信号，通过在天线孔径前放置金属板所产生的反射波来确定。因此，可以从观测信号中剔除两个以上天线之间的直线路径所对应的特定时间间隔的信号，得到频域的投影数据。在1 GHz到4 GHz的频率范围内，每500 MHz重复此图像重建过程。主要的差异是观察到的信噪比和数量的不可避免的人为干扰造成的绕射和透射波之间的干扰。在1 GHz带宽的图像重构中，利用1 GHz ~ 2 GHz频率分量重构的图像质量最优。结果表明，设计在1 ~ 2 GHz频段的CP-MCT是合理的。TD-MCT的空间分辨率估计值与CP-MCT.2相同。时域微波计算机断层扫描的实际意义：一般来说，任何微波计算机断层扫描的扫描仪都必须在盐水溶液中进行测量，将天线和生物物体浸入其中，进行阻抗匹配。在微波成像频率范围内，盐水溶液的介电常数值与生物组织相近。尽可能增大信号的带宽，才能最有效地发挥时域测量的优势。另一方面，由于层析测量的传输损耗随着频率的增加而急剧增加，因此很难增加频率。在盐水溶液中也很难实现带宽非常宽的天线，例如超过3ghz。CP-MCT在频域提取透射波的直路传输分量，TD-MCT在时域提取直路传输分量。然而，CP-MCT可以通过测量低于1khz或更低的极低频信号来做到这一点。与时域的测量过程相比，这是一项简单的任务。认为只有光谱信息得到有效利用，TD-MCT的优势才会显现出来。少

1. Conclusions of the study : Feasibility of microwave imaging using measurement techniques in time domain has been investigated. Time domains measurement techniques are used in this study for discriminating the wave components that transmitted on the straight path between two antennas from the scattered- or reflected wave components. This is a similar technique to CP-MCT in which the straight path between two antennas is discriminated by use of the techniques in frequency domain measurement of a chirp radar.The developed experimental system of Time Domain Microwave Computed Tomography (TD-MCT) that is consisted of a vector network analyzer and microwave scanner and measures S21 in the frequency range from 0.8 GHz to 4.2 GHz. Transmission time between two antennas can be time domain signal is determined by the reflected waves made by placing a metal plate in front of the antenna aperture. Accordingly, the signal which has a specific time interval corresponding to the straight path betw … More een two antennas can be excised from the observed signal to make projection data in frequency domain. This image reconstruction procedure is repeated at every 500 MHz in the frequency range from 1 GHz to 4 GHz. The major differences were observed in the signal to noise ratio and amount of unavoidable artifacts caused by interference among the diffracted- and transmitted-waves. In image reconstruction with 1 GHz bandwidth, the image reconstructed by use of the frequency components from 1 GHz to 2 GHz exhibits the most excellent quality. It is concluded that the CP-MCT that was designed at 1 GHz to 2 GHz has been properly designed. The estimated spatial resolution of TD-MCT shows the same value with that of CP-MCT.2. Practical significance of time domain microwave computed tomography : Generally speaking, any scanners of microwave computed tomography have to make measurement in bolus saline solution by immersing the antennas and biological object for impedance matching between them. Saline solution shows similar permittivity values with biological tissues in the frequency range of microwave imaging. The advantages of time domain measurement are derived most effectively by increasing the bandwidth of the signal as much as possible. On the other hand, it is hard to increase the frequency of tomographic measurement because transmission loss is increased abruptly when frequency increases. It is also hard to realize the antenna with a very wide bandwidth, for example more than 3 GHz in saline solution. CP-MCT extracts straight path transmission components from transmitted waves in frequency domain and TD-MCT does the same thing in time domain. However, CP-MCT can do it by measuring very low frequency signal which is lower than 1 kHz or less. This is an easy task as compared to the measurement procedure in time domain. It is considered that the advantages of TD-MCT will be found when spectral information can be used effectively. Less